1. Field of the Invention
The present invention is directed towards a biodegradable drilling mud and to a process for its preparation, irrespective of its use, on land or offshore. More particularly, the invention relates to drilling muds used on offshore platforms, the cuttings from which can be discharged at sea without treatment and which must, therefore, not present a major pollution hazard with respect to the marine environment.
2. Description of the Background
Drilling mud plays an essential role during drilling; it ensures maintenance of the fluids, gases, water and oils in the terrains passed through, the raising of the cuttings from the drilled terrain, the lubrication and cooling of the drilling tool, the maintenance of the cuttings in suspension and their raising to the surface. On leaving the well, it is necessary to separate the drilled cuttings from the mud before recirculating it in the well.
In parallel with this recirculation of the mud is the problem of storage of the mud-contaminated cuttings. Although this is difficult on a land platform, it is much more difficult to solve on an offshore platform, since the constraints regarding environmental regulation are much stricter. Thus, discharging the cuttings at sea is generally not authorized since, even after separation from the mud, these cuttings constitute a source of marine pollution and must be returned to land to be processed, which is very expensive for the operator.
Discharge of the cuttings at sea may depend essentially on the nature of the mud used, which is toxic or biodegradable to a greater or lesser extent.
Different types of muds are used for drilling. They are, firstly, water-based muds whose main organic fraction is based on cellulose, cellulose derivatives and acrylic acid derivatives. These muds are easily biodegradable and do not give rise to any potential degradation of the environment. Oil-based muds are preferable to the latter since they can be recovered and they give technically better performance. They have far superior lubricating power, which is most particularly favourable in the context of deviated wells. They withstand high temperature better, in particular a temperature above 150xc2x0 C., and, lastly, they are inert with respect to reactive formations such as clay formations. They consist essentially of a stabilized reverse emulsion of water in oil, generally of gas oil or a mineral oil, and their physicochemical properties are adjusted by means of viscosifying additives, weighting additives and the like. However, they represent a very great risk of pollution since they are toxic and difficult to biodegrade. A new generation of oil-based muds, also known as replacement muds, is used as a replacement for muds based on gas oil, but they are still considered as being too polluting to authorize discharge of the cuttings at sea. These muds are formulated from polyglycols, polyethylene glycol or poly(xcex1-olefins) as stabilized emulsions. Although these muds were developed to replace gas oil and mineral oils, rich in aromatic compounds in the formulation of oil-based muds, and although they have physicochemical properties similar to those of gas oil or mineral oils without being toxic, they are nevertheless slow to biodegrade or relatively poor at doing so.
In the current state of the legislations in force in certain countries, discharge offshore of. cuttings containing oil-based muds or replacement muds, with the exception of esters, is banned since these muds are not considered as biodegradable.
The biodegradability of the ester-based muds used hitherto also as replacement muds has also been demonstrated (see the publication by J. Steber, C. P. Herold (Henkel KGaA) and J. M. Limia (Baroid) Offshore, 60, September 1984). However, although the biodegradation processing proposed is effective, it is very long, which necessitates storage of the cuttings which is not easy to achieve on offshore platforms on which space is limited.
To accelerate the biodegradation of the muds, whether they are oil-based muds or ester-based replacement muds, it is possible to make them undergo enzymatic treatment prior to the bacteriological treatment which may be performed.
Enzymatic treatments of muds in general are known in particular to increase their permeability in the formation at the bottom of a well, as described in German patent DD 240,905 dated Sep. 13, 1985. Other treatments, as described in U.S. Pat. No. 5,126,051, use enzymatic hydrolysis reactions by means of cellulases or derivatives thereof, in order to degrade the residual viscosifying compounds from the mud.
In European Patent No. EP 0 712 809, to accelerate the biodegradation of the discharges and of the mud itself, the Applicant has recommended chemically modifying the ester-based muds in order to accelerate their bacteriological degradation by hydrolyzing the said mud using a lipase at basic pH, preferably ranging from 9 to 10, and at a temperature below 60xc2x0 C.
Similarly, in International Application No. PCT/FR97/02354 filed Dec. 18, 1997, the Applicant has recommended chemically modifying replacement muds based on olefins or polyolefins by oxidizing them before discharge, by means of an oxidizing anion at acidic pH, preferably below 4, prior to the bacteriological treatment which may be performed.
Accordingly, one object of the present invention is to provide a biodegradable drilling mud whose composition whose physicochemical properties are comparable with those required for a drilling mud, and whose composition is readily biodegradable.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a biodegradable drilling mud consisting of a water/organic phase reverse emulsion in a ratio by volume ranging from 50/50 and 10/90, comprising weighting additives, viscosifying additives, filtrate reducers, emulsifiers and any other additive intended to achieve the desired physicochemical properties, characterized in that it contains, for the purpose of biodegrading it,
a) from 1 to 5% by volume, relative to the volume of the organic phase without additive, of a nitrogen compound from the group consisting of amino acids, carbamic mono- and polyamides and mono- and polyacarbamic amides, of formulae (I) and (II) below, respectively:
Rxe2x80x94[NHxe2x80x94CH2]n-COxe2x80x94NH2xe2x80x83xe2x80x83(I)
Rxe2x80x94NHxe2x80x94[COxe2x80x94NH]m-Hxe2x80x83xe2x80x83(II)
xe2x80x83with R being hydrogen or an alkyl group comprising from 1 to 10 carbon atoms, n and m being integers ranging from 1 to 5,
b) from 2 to 10% by volume, relative to the volume of the organic phase without additive, of at least one phosphorus compound from the group of surfactant phosphorus compounds, preferably an alkyl or alkenyl phosphoric ester comprising from 10 to 15 carbon atoms,
c) and from 2 to 10%, relative to the volume of the organic phase without additive, of at least one carboxylic compound from the group consisting of fatty acids, fatty acid esters of fatty alcohols comprising carbon-based chains comprising from 12 to 24 carbon atoms, alone or as a mixture, and plant or animal oils.